(4) D. C. S. asks for a good recipe for cleaning and polishing dirty and tarnished brass. A. Dip for a short time in strong hot aqueous solution of caustic alkali, rinse in water, dip for a few moments in nitric acid diluted with an equal volume of water, rinse again, and finish with whiting. (5) C. J. H. asks for the simplest way of producing a coating of the magnetic or black oxide of iron on iron plates 3 feet x 6 feet. I think it is called the Barff process. A. See pp. 1041 SCIENTIFIC AMERICAN SUPPLEMENT, and 232, vol. 36, and 4, vol. 37, of the SCIENTIFIC AMERICAN. How can I make tissue paper impervious to air and water, and yet strong enough to confine gas? A. You may pass the fabric through a solution of about 1 part caoutchouc in 35 parts of carbonic disulphide, exposing it then to the air until the solvent has evaporated. (6) J. H. J. asks how to use hyposulphite (?) of soda to neutralize chloride of lime in cotton and linen goods after bleaching the same. A. After washing from it the large excess of the hypochlorite, the fabric is passed slowly through a solution containing about 10 per cent of the hyposulphite, and then again thoroughly washed in clean water. (7) Columbus asks for a recipe for making ink to rule faint lines, such as he is now writing on. He wants it to rule unit columns in books. A. Dissolve in a small quantity of warm water 20 parts of Prussian blue by the aid of 3 parts of potassium ferrocyanide, and dilute the solution with thin gum water until the proper degree of color is obtained. (8) A. I. B. asks: Can I add anything to Arnold's writing fluid which will cause it to give a good free copy in my letter book? A. Try a little sugar. (9) R. & C. ask for information in regard to the process of printing copies of drawings made on transparent materials, by using chemically prepared paper and exposing to the sunlight. A. It is based on the fact that an acid in the presence of potassium dichromate strikes a blackish-green color when brought in contact with aniline. The paper is prepared by floating it on a bath of aqueous solution of potassium dichromate and a trace of phosphoric acid, and then drying it in the dark. Aniline is dissolved in a little alcohol, and the mixed vapors allowed to come into contact with the sensitive paper that has been exposed to strong sunlight beneath the drawing, when the portions not changed by the sunlight assume the dark color mentioned. All that is requisite is that the paper or cloth original should be fairly penetrable by the light. A piece of paper sensitized as indicated, a sheet of glass to place over the drawing, and a box in which to place the exposed print to the aniline vapor are the only necessary plant.